This invention relates in general to an improvement in a dual-shaft gas turbine engine having two independently rotatable shafts on which a gasifier turbine and a power turbine are respectively mounted, and more particularly to a device for controlling fuel supply at engine starting of the dual-shaft gas turbine engine.
In a dual-shaft gas turbine engine, fuel supply under normal engine operating conditions is usually controlled in a so-called proportional or proportional-integral closed loop, i.e., feedback, control mode so that the rotational speed of a power turbine agrees with a target level. With such a fuel supply control mode, a suitable fuel supply cannot be expected during engine starting. The power turbine rotational speed deviates from the normal speed target level extensively during engine starting. Therefore a large amount of feedback control is required during starting; this large amount of feedback control results in excessive fuel consumption.
As appreciated, it is desirable to obtain an optimum air-fuel ratio during engine starting, and accordingly an optimum amount of fuel for ignition is supplied in dependence on an amount of air corresponding to the rotational speed of a compressor turbine which is driven by a starter motor. Therefore, during engine starting, it is not suitable to provide the normal speed closed loop mode control of the rotational speed of the power turbine so the turbine speed agrees with the normal speed target level.
In this regard, control of power turbine rotational speed during engine starting has been previously provided from experience by determining optimum fuel amount for engine starting. The thus determined fuel amount is supplied to the engine until the power turbine rotational speed reaches a predetermined level after engine ignition. Then the closed loop control is initiated.
Because the optimum fuel supply for engine starting is maintained until the power turbine rotational speed reaches the predetermined level, there is a tendency for a prolonged starting time period. Therefore, such a control mode is not particularly suitable for a drive power source used for driving an emergency electric power generating device which requires a shorter engine starting time period.